Tests will vary on how soon before your missed period you can take them, and also if you need to use the first urine of the day so read the instructions before you take it so you know you're getting an accurate result.

Some of them tell you to use the furst urine of the day because this will contain the highest levels of the pregnancy hormone.

Try a few days after your missed period. Test in the morning as the hormone that a pregnancy test is looking for, hCG, will be at it's most concentrated (as you haven't drunk anything during the night to dilute it further). If it's negative but you still haven't got your period after a week or so, do another test.

False negatives are reletivly common becuase not enough hormone has been released. If you've got a choice of pregnancy tests to choose, go for one with the greatest sensitivity (e.g. 20 UI/ml hCG as opposed to some which are 50+ requiring more of the hormone to show up as positive)